<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title></title>
    <script src="https://cdn.bootcdn.net/ajax/libs/vue/2.6.9/vue.js" type="text/javascript" charset="utf-8"></script>
</head>

<body>
    <div id="app" style=" text-align: center;">
        <button @click="sub">-</button>
        <span>{{num}}</span>
        <button @click="sup">+</button>
    </div>

    <script type="text/javascript">
        var vm = new Vue({
            el: "#app",
            data: {
                num: 1
            },
            methods: {
                sub: () => this.vm.num--,
                sup: () => this.vm.num++
            }
        })
    </script>
</body>

</html>